Field of the Invention
The present invention relates to a method of manufacturing a silicon carbide semiconductor device, and more specifically relates to a method of manufacturing a silicon carbide semiconductor device, such as a silicon carbide Schottky barrier diode, which uses a rectifying action of a Schottky barrier existing at a junction interface between a semiconductor and a metal.
Description of the Background Art
A diode is an electronic component that restricts a direction in which a current flows. The diode allows the current to flow in one direction, and prevent the current to flow in the opposite direction. Accordingly, the diode can be considered as an electronic version of a mechanical one-way valve. A circuit that requires a current to flow only in one direction includes one or more diodes.
A Schottky barrier diode (hereinafter, sometimes referred to as “Schottky diode”) is a diode formed by contact between a semiconductor layer and a metal layer, which is different from a pn-junction diode formed by contact between two dissimilar semiconductor layers.
A silicon carbide (SiC) semiconductor is a Group IV-IV semiconductor which is very stable, and has a feature of a wider bandgap, a higher thermal conductivity, and a higher breakdown electric field than those of a silicon (Si) semiconductor. Therefore, a silicon carbide semiconductor device including SiC is operable under a high-temperature condition, and thus has been attracting attention as a high-power device with a high breakdown voltage and a low loss.
In preparation of a Schottky diode having a rectifying action obtained by a junction barrier of silicon carbide and a metal, a selection of a Schottky metal material and stabilization of reverse characteristics thereof are important factors. As a Schottky metal material, titanium (Ti), nickel (Ni), molybdenum (Mo), tungsten (W), or the like, is adopted.
In a case where a material causing a Schottky barrier is made a contact with a semiconductor, if a heat release is predominant, the current density J is represented by the following expression (1).J=J0{exp(qV/kT)−1}  (1)
Here, J0 represents a saturation current density, q represents a charge amount, V represents an applied voltage, k represents Boltzmann's constant, and T represents the temperature. The saturation current density J0 is represented by the following expression (2).J0=A*T2exp{−q(φb)/kT}  (2)
Here, φb represents a Schottky barrier height, and A* represents Richardson's constant.
As apparent from the expression (2), as the Schottky barrier height φb increases, the saturation current density J0 decreases so that a reverse current can be suppressed.
However, in a conventional film formation method in Schottky junction between a Schottky metal layer and a silicon carbide substrate, an actual Schottky barrier height φb is lower than a theoretical Schottky barrier height φb due to an influence of damage to a junction interface, such as a disordered lattice, occurring in the process. For example, while the theoretical Schottky barrier height φb is 1.25 eV, the actual Schottky barrier height φb has a lower value of about 1.19 eV to 1.23 eV, and additionally has a variability in a wafer plane. There has been a problem that this variability causes a large variability between reverse current characteristics and forward current characteristics in the wafer plane and thus the reverse current characteristics and the forward current characteristics are not stabilized.
Techniques for suppressing damage to the junction interface occurring in the process are disclosed in, for example, Japanese Patent Application Laid-Open No. 10-308358 (1998) and Japanese Patent Application Laid-Open No. 2008-103705. In the technique disclosed in Japanese Patent Application Laid-Open No. 10-308358 (1998), to prevent entry of sputtering damage to a semiconductor layer, a metal film is vapor-deposited through a thermal vapor-deposition process, and then a metal film is formed thereon through a sputtering method (for example, see Japanese Patent Application Laid-Open No. 10-308358 (1998) (page 3)). In the technique disclosed in Japanese Patent Application Laid-Open No. 2008-103705, to reduce sputtering damage that occurs in causing a metal film to be deposited through a direct current (DC) sputtering method, DC power is controlled (for example, see Japanese Patent Application Laid-Open No. 2008-103705 (page 6)).
Additionally, Japanese Patent Application Laid-Open No. 9-129901 (1997) discloses a technique in which the Schottky barrier height is controlled by controlling the flatness of an interface between an electrode metal and a semiconductor. In the technique disclosed in Japanese Patent Application Laid-Open No. 9-129901 (1997), a process for flattening a surface of a semiconductor substrate is performed before the electrode metal is vapor-deposited (for example, see Japanese Patent Application Laid-Open No. 9-129901 (1997) (pages 7-8)).
Among device characteristics of a silicon carbide Schottky barrier diode, the reverse current characteristics and the breakdown voltage characteristics are greatly influenced by wafer defects, epitaxial layer defects, and process defects. Moreover, the forward current characteristics are greatly influenced by pretreatment conditions under which Schottky junction is formed and film formation conditions under which a Schottky metal film is formed.
As described above, in the method of manufacturing the silicon carbide semiconductor device according to the conventional techniques, there has been the problem that the Schottky barrier height φb has a variability in the wafer plane so that the reverse current characteristics and the forward current characteristics are not stabilized.
Techniques for solving such a problem include the techniques disclosed in Japanese Patent Application Laid-Open No. 10-308358 (1998), Japanese Patent Application Laid-Open No. 2008-103705, and Japanese Patent Application Laid-Open No. 9-129901 (1997), mentioned above. However, the technique disclosed in Japanese Patent Application Laid-Open No. 10-308358 (1998) is not a technique for improving the sputtering method itself. Therefore, even though the technique disclosed in Japanese Patent Application Laid-Open No. 10-308358 (1998) is used, damage occurring when a metal film is formed by the sputtering method cannot be reduced.
In the technique disclosed in Japanese Patent Application Laid-Open No. 2008-103705, the DC sputtering method is adopted. However, in the DC sputtering method, a plasma is formed near a specimen. Therefore, a silicon carbide wafer that is the specimen may be damaged.
The technique disclosed in Japanese Patent Application Laid-Open No. 9-129901 (1997) is a technique relating to a pretreatment performed prior to the formation of a metal film that will be an electrode, and not a technique relating to the formation of an electrode. Damage occurring when an electrode is formed by a sputtering method cannot be reduced by adoption of the technique disclosed in Japanese Patent Application Laid-Open No. 9-129901 (1997).